A leading blindness charity is claiming the son of its billionaire founder won’t fork over a promised $4 million donation.

ORBIS International has sued James Ueltschi, who lives in a waterfront mansion in Vero Beach, Fla., claiming he is withholding the cash from the New York-based charity, which sends its Flying Eye Hospital jet around the world to perform free eye surgeries.

James, 63, manages the estate of his late father, aviation pioneer Albert Ueltschi, who died in October with a net worth of $2.1 billion.

In a March 2007 ceremony attended by Mayor Bloomberg and Ethiopian President Girma Wolde-Giorgis, Albert Ueltschi announced a $10 million gift to ORBIS to help replace its aging jet, which is equipped with an operating room.

The charity collected $6 million, but James “has not paid the final installment of the Lead Gift,” papers filed in Manhattan civil court allege.

ORBIS also filed a $4 million claim Wednesday against Ueltschi’s estate in a Florida probate court.

ORBIS took in $83.9 million in contributions in 2011. Records show it has $81.5 million in expenses.